I could have made references to Figure Numbers - such as (163) - into "links".
I recommend instead that when you want to look at a drawing on line that you
start another "instance" of your browser, and while in that browser go the
Appendix: Drawings and use that
"instance" to access the drawings. There are many advantages using this
method for such large files.

The drawing marked Modern Timing
in "Appendix: Drawings" (above) is a modern drawing used to run the
current machine. Small changes had to be made from Babbage's original
timing in light of the physical constraints of allowable acceleration.

Converter's comments

I sympathize with the problem of reproducing Babbage's drawings in the book.
The original drawings were
large, but to make a book that is reasonable to manufacture, there is great
pressure to make each drawing sheet fit on a book page. Manufacturing and reading a
document with "oversize" fold out pages is not pleasant. Unfortunately there
is a limit to the resolution (dots/length) of printing and Xeroxing.

The Babbage drawings were scaled down to fit the 8-3/16 by 11-11/16 inch
page of the manual. I scanned the drawings at 300 dots per inch (118 dots per cm)
which is the effective limit of the drawings as printed.

At some future date I may be able to present more readable drawings :-)

I was reading the comments at the end of "Charles Babbage's Difference
Engine No. 2 Technical Description" by Mr. Swade. The answer to why Mr
Swade description appears to be in error is that both Alain Calzas and
Jan-willem De Bleser assume all the columns are digit wheels are
numbered in the same manner (all increasing as they rotate clockwise).
This is not true and can be seen in many of the videos available on
youtube. The numbering alternates from increasing in the clockwise
direction to increasing counterclockwise direction for each column from
left to right. This is clearly shown in Tim Robinson's video of his
meccano version named "part 2 front close up".
So Mr Swade's description is exact.
Marc LaViolette
Assistant Professor
Dept. Mechanical and Aeronautical Engineering
Royal Military College of Canada
Kingston, Ontario, Canada

Jan-willem De Bleser wrote:
> That would most certainly be another valid way to design it. To know
> which was Babbage's design you'd have to study his timing diagram -
> something I haven't found the time for yet.
>
His timing diagram (which has some errors) definitely assumes the
alternating columns. The reason is that with alternating columns you
can overlap the restoring of the source wheels with the carry operation
on the result wheels. If instead you made all the columns in the same
order then you would have to transfer the value to the sector from the
source, restore source and add, and then finally carry, so all these
three would have to be sequential and it would slow the cycle down.
Since Babbage was quite obsessed with speed, it seems very unlikely he
would have done this just to avoid mirroring the columns. However the
timing diagram and the details of the cam profiles make it clear he
intended to use the alternating column approach.
> You say this reversed setup is how the actual machine in the Science
> Museum is built? Have never seen it myself, sadly enough.
>
Yes, they did make this change. In my own model I followed the same
path, however I did find some further optimizations which reduced the
number of independent controls from what Babbage used. I don't know if
he simply overlooked these. More likely he chose not to exploit them,
preferring to keep the flexibility of fully "horizontal" control. He
may also have been concerned about uneven wear from the greater load
which some control paths would have to take with the optimizations in place.
Tim Robinson

I am no expert on the difference engine but I believe that what Alain
states is correct - his explanation is in fact one way it would work as
drawn.
Consider Doron's description of the "Giving Off" phase:
1) The sector wheel is fully engaged and the right hand drive axis is
lowered.
2) The right axis is rotated 81 degrees clockwise, reducing the count to
zero and driving the left wheel via the sector wheel.
3) The right wheel is now zero and the left wheel is the sum of the
original value and the right value.
There is one inconsistency in this explanation: both figure wheels are
rotating in the same direction and thus must both be decremented or
incremented, assuming the same direction of numbering. This means that it
is impossible for the right wheel to be reduced to zero while
simultaneously increasing the value of the left wheel. Alain's solution
solves this, as first the right figure wheel is reduced in value, followed
by both figure wheels being increased in value.
Jw